(a) Field of the Invention
The present invention relates to a method for preparing uniform metal oxide nanoparticles with high reproducibility. More specifically, the present invention relates to a method for preparing metal oxide nanoparticles having improved uniformity of nanoparticles and reproducibility of a process.
This application claims priority to Korean Patent Application No. 10-2013-0151760 filed on Dec. 6, 2013 with the Korean Intellectual Property Office, the content of which is herein incorporated by reference in its entirety.
(b) Description of the Related Art
Nanoparticles can have application in a wide range of uses, such as a nano-electronic fusion technology, a biological imaging technology and a medical field and thus they have drawn much interest in the academic and the industry. In particular, since superparamagnetic iron oxide nanoparticles are used in various fields such as a magnetic resonance imaging (MRI) contrast agent, a treatment on a cellular level, a hyperthermia, a drug delivery, a cell separation, and a nucleic acid separation, they account for a large portion of the various types of nanoparticles.
In order to enhance the utilization of iron oxide nanoparticles and reinforce the inherent characteristics of nanoparticles, it is important to synthesize uniform nanoparticles with the desired size in large quantities. Also, in order to produce these high-quality nanoparticles economically, it is important to reduce both process variables and production costs by using a simple production process.
However, in the case of the methods that are currently commercialized, nanoparticles are largely synthesized in a water system or in a gaseous state. The nanoparticles thus obtained are usually hard to have uniform particle shapes and they may have reduced crystallinity. Also, the production of uniformly sized nanoparticles is not easy, and the adjustment of their size is difficult.
Recently, many researchers have developed new processes for preparing, in an organic solvent system, metal oxide nanoparticles having a relatively high quality, that is, uniform size and crystallinity, as compared with the nanoparticles synthesized in a conventional water system.
Prof. Taeghwan Hyeon's research group published in 2004 the results of their research about the synthesis of metal oxide nanoparticles from organic phase using metal chloride as a starting material (Nature Materials 3, 891 895 (2004)). The process for preparing nanoparticles proposed by this research group is composed of a first reaction step of reacting a metal chloride and sodium oleate to give a metal-oleic acid precursor, and a second reaction step of thermally decomposing the precursor to form a nanoparticle. The above reaction allows the synthesis of uniformly sized nanoparticles by using oleic acid as a dispersion stabilizer in an organic solvent having a high boiling point.
The V. Colvin group of Rice University in the United States published in 2004 the results of their research about the synthesis of iron oxide nanoparticles using iron hydroxide as a starting material (Chem. Commun, 2306-2307 (2004)) and found that the nanoparticles are formed by mixing iron hydroxide (FeOOH) with oleic acid in a high boiling point solvent and then heating the mixture at high temperature.
However, the above-mentioned methods for preparing uniform nanoparticles have a problem that the process conditions are complicated and the reproducibility is declined. Among the above methods, the preparation method of nanoparticles proposed by Prof. Taeghwan Hyeon's research group further includes an additional reaction step of preparing a precursor as compared to the production method proposed by Colvin group, and a separation process of recovering the precursor in the first reaction step is additionally necessary. Therefore, production costs due to the addition of process are increased.
In the case of the preparation method of nanoparticles proposed by Colvin group, there were problems that the reproducibility between batches and the dispersibility of the size distribution within batches are increased, and the nanoparticles are not well formed. The problem of the method proposed by Colvin group is due to water generated by decomposition of iron hydroxide which is a precursor in the formation of nanoparticles. Iron hydroxide is decomposed to form water. The water is vaporized in a high-temperature organic solvent and then liquefied at the top of the reactor, and re-flowed into the reactor. When the liquefied water is re-flowed into the reactor, a rapid vaporation occurs, which causes a partial temperature change of the reaction solution and an instantaneous pressure increase in the reactor. The temperature of the reaction solution is an important factor in maintaining the size and uniformity of the particles. And a rapid increase in pressure inside the reactor becomes a threat to safety. In the case of small-scale reaction, the above two can be ignored. However, in the case of large-scale reaction, the amount of water generated increases, which deviates from the level that can be ignored anymore.